The Rho GTPases regulate diverse biological processes such as growth control and cytoskeleton organization. Recently we demonstrated that WASP, the protein that is defective in the Wiskott-Aldrich syndrome, is a novel effector for the GTPase CDC42Hs and showed that it is involved in actin polymerization. Cells from WAS patients show several cytoskeletal abnormalities including abnormal shape and aggregation of platelets; and abnormal shape and function of microvilli on T and B cells. The immune defects in WAS patients are thought to be due to failure of T and B cells to respond to activating stimuli, best characterized as a failure to produce IL-2 and proliferate in response to anti-CD3 stimulation. In this proposal, we will seek to understand how WASP receives and then transmits signals initiated at the antigen receptor of T lymphocytes. We will define the receptors and signaling molecules leading to the activation of the CDC42Hs and the function of WASP. We will identify the down stream effector molecules of WASP that control actin polymerization and also determine if actin polymerization is required for transmission of signals to the nucleus. We will define the specific targets of WASP that are required for transcriptional activation and then determine if actin polymerization or some other aspect of WASP function is required for these effects. We will define what aspect of the control of expression of early immune response genes such as IL-2 and CD40L is affected by mutations in WASP, and determine how WASP effects this specific aspect of gene expression. At the end of the 5 year grant period we hope to have a complete molecule-by-molecule description of the events transmitting signals from the T cell antigen receptor to the WAS protein to the IL-2 gene. Delineation of this pathway will contribute to our understanding of how cytoskeletal events are integrated with transcriptional activation and will help to understand an important genetic human disease.